1. Field of the Invention
This invention relates generally to optical temperature compensation of spectral modulation sensors, and, more particularly, to ratiometric interrogation of spectral modulation sensors.
2. Description of the Related Art
In advanced aircraft applications, the use of fiber optic sensors to carry information between sensors and control modules provides immunity from contamination by electromagnetic sources, reduces volume and weight by eliminating the need for electromagnetic shielding, and further reduces weight by replacing metal conductors with low weight optical fibers. Advantages afforded by spectral modulation based fiber optic sensors include polarization independence, ease of multiplexing sensors with a common electro-optical interface, and reduced sensitivity to variations of link losses.
The sensitivity of these sensors to source temperature, however, severely limits their usefulness for aircraft engine or airframe applications. Typically, light emitting diodes (LEDs) are used for interrogating the sensors. The source temperature drift is critical in view of the high temperatures and acute temperature changes that occur in aircraft engines. The process of using thermo-electric coolers for controlling the source temperature is slow and breaks down at high temperatures due to the diffusion of carriers and electro-migration in the thermo-electric element.
To avoid measurement inaccuracies resulting from changes in the light source intensity and changes in light transmission intensity due to bending of the optical fibers or optical connector loss, Saaski et al., U.S. Pat. No. 4,945,230, issued Jul. 31, 1990, describes a technique using ratiometric measurement with spectral modulation sensors (SMS). In the Saaski device the physical parameter being measured causes changes in the reflectivity and transmission of the sensor's optically resonant structure and thus spectrally modulates the output light from the sensor as a function of the physical parameter being measured. The spectrally modulated output light is converted into an output electrical signal by detection means. In one embodiment the detection means splits the spectrally modulated light into two spectral components, each of which is separately converted into an electrical signal by a photodetector means. A divider circuit then takes the ratio of these two electrical signals to provide an output signal. The Saaski device does not avoid inaccuracies due to source temperature.